WASHINGTON — Television ads sponsored by the oil industry’s main trade association are being broadcast in five U.S. states to pressure Democrats to back the proposed Keystone XL pipeline, part of a flurry of last-minute lobbying before a Senate vote as soon as this week.

The American Petroleum Institute, a Washington-based group that represents oil companies including Exxon Mobil Corp. and Chevron Corp., said Monday that the ads will appear in Colorado, Delaware, Minnesota, New Mexico and South Dakota and run until May 7.

Environmental groups are mounting their own efforts, including organizing phone calls and writing letters to urge opposition to the pipeline that would connect Canadian oil with U.S. refineries on the Gulf Coast.

The API-backed ads note former presidents George W. Bush and Bill Clinton, as well as investor Warren Buffett, support Keystone XL.

“We’re looking at anyone who could be supportive,” Cindy Schild, downstream operations senior manager at API, said today on a call with reporters.

Keystone XL is “not only in the national interest, but it also serves their political interest” to approve the legislation, Schild said. The bill, pushed by Senators Mary Landrieu, a Louisiana Democrat, and John Hoeven, a North Dakota Republican, would bypass U.S. President Barack Obama and approve the project.

The Senate could vote this week on Keystone XL, either as an amendment to an energy efficiency bill or as separate legislation. Either way, supporters last week said they were short of the 60 votes necessary to clear a procedural hurdle and advance the bill.

Keystone XL, proposed by Calgary-based TransCanada Corp., would connect Canadian heavy crude with refineries in Texas and Louisiana.

The U.S. State Department last month delayed its review of the project until a legal challenge to the existing route in Nebraska is resolved. A decision isn’t likely before the November election.

Supporters of Keystone XL say the delay is less about the court case and more about letting Obama avoid deciding an issue that splits two Democratic constituencies — environmental groups and labour unions — before the congressional election.

Environmental groups including the Sierra Club in San Francisco and the Natural Resources Defense Council in New York said they were organizing phone calls, letter-writing campaigns and protests to encourage senators to block the legislation.

They view Keystone XL as a threat to the climate because it would encourage development of Alberta’s oilsands, which releases more greenhouse gases than other forms of oil.

Jason Kowalski, policy director for 350.org, which lobbies for action to combat climate change, said in a statement that it was “launching a full-court press to convince swing state senators to come out against the pipeline.”